This invention relates to (i) a polymer having a specific structure, (ii) a resist composition comprising the polymer as a base resin, and (iii) a patterning process using the resist composition.
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 xcexcm or less.
For resist materials for use with a KrF excimer lasers, polyhydroxystyrene having a practical level of transparency and etching resistance is, in fact, a standard base resin. For resist materials for use with ArF excimer lasers, polyacrylic or polymethacrylic acid derivatives and polymers containing aliphatic cyclic compounds in the backbone are under investigation. All these polymers have advantages and disadvantages, and none of them have been established as the standard base resin.
More particularly, resist compositions using derivatives of polyacrylic or polymethacrylic acid have the advantages of high reactivity of acid-decomposable groups and good substrate adhesion and give relatively satisfactory results with respect to sensitivity and resolution, but have extremely low etching resistance and are impractical because the resin backbone is weak. On the other hand, resist compositions using polymers containing alicyclic compounds in their backbone have a practically acceptable level of etching resistance because the resin backbone is robust, but are very low in sensitivity and resolution because the reactivity of acid-decomposable protective groups is extremely low as compared with those on the (meth)acrylic polymers. Since the backbone of the resin is too robust, substrate adhesion is poor. These compositions are thus impractical as well.
Both the (meth)acrylic and alicyclic backbone systems commonly have the problem of pattern disruption due to swelling of resist film. Resist compositions based on these systems have been designed so as to improve their resolution performance by increasing the difference in dissolution rate before and after exposure, and as a consequence, they eventually become highly hydrophobic. Highly hydrophobic resist compositions, when applied as a film and processed with a developer, can maintain the film tenaciously in unexposed regions and allow the film to be instantaneously dissolved in over-exposed regions, while relatively broad exposed regions therebetween allow penetration of the developer, but are kept undissolved, that is, swollen. At the very small pattern size for which ArF excimer laser is actually used, those resist compositions which allow adjacent pattern strips to be joined together and disrupted on account of swelling are rejected. While a finer pattern rule is being demanded, there is a need to have a resist material which is not only satisfactory in sensitivity, resolution, and etching resistance, but fully restrained from swelling.
Therefore, an object of the present invention is to provide (i) a polymer having improved reactivity, robustness and substrate adhesion as well as minimized swell during development, (ii) a resist composition comprising the polymer as a base resin, which has a higher resolution and etching resistance than conventional resist compositions, and (iii) a patterning process using the resist composition.
It has been found that novel polymers comprising recurring units of the following general formula (1) and having a weight average molecular weight of 1,000 to 500,000, which are produced by the method to be described later, have improved reactivity, robustness or rigidity and substrate adhesion as well as an appropriately high solubility and minimized swell in a developer; that a resist composition comprising the polymer as the base resin has a high resolution and etching resistance; and that this resist composition lends itself to precise micropatterning.
In a first aspect, the invention provides a polymer comprising recurring units of the following general formula (1) and having a weight average molecular weight of 1,000 to 500,000: 
Herein R1 and R2 each are hydrogen or methyl; R3 and R4 each are hydrogen or a straight, branched or cyclic, monovalent hydrocarbon group of 1 to 15 carbon atoms which may contain a hetero atom, and R3 and R4 may bond together to form a ring, wherein R3 and R4 together represent a straight, branched or cyclic, divalent hydrocarbon group of 1 to 15 carbon atoms which may contain a hetero atom; each of R5 to R8 is hydrogen, a hydroxyl group or a straight, branched or cyclic, monovalent hydrocarbon group of 1 to 15 carbon atoms which may contain a hetero atom, at least one of R5 to R8 contains a hetero atom, any two of R5 to R8 may bond together to form a ring, wherein the ring-forming two R""s together represent a straight, branched or cyclic, divalent hydrocarbon group of 1 to 15 carbon atoms which may contain a hetero atom; R9 and R10 each are hydrogen or methyl; each of R11 to R14 is hydrogen or a straight, branched or cyclic, monovalent hydrocarbon group of 1 to 15 carbon atoms which may contain a hetero atom, a pair of R11 and R12, a pair of R11 or R12 and R13, a pair of R11 and R14, or a pair of R13 and R14 may bond together to form a ring, wherein each pair represents a straight, branched or cyclic, divalent hydrocarbon group of 1 to 15 carbon atoms which may contain a hetero atom; R15 is hydrogen, methyl or CH2CO2R17; R15xe2x80x2 is hydrogen, methyl or CH2CO2R17xe2x80x2; R15xe2x80x3 is hydrogen, methyl or CH2CO2R17xe2x80x3; R15xe2x80x3xe2x80x2 is hydrogen, methyl or CH2CO2R17xe2x80x3xe2x80x2; R16 is hydrogen, methyl or CO2 R17; R16xe2x80x2 is hydrogen, methyl or CO2R17xe2x80x2; R16xe2x80x3 is hydrogen, methyl or CO2R17xe2x80x3; R16xe2x80x3xe2x80x2 is hydrogen, methyl or CO2R17xe2x80x3xe2x80x2; R17, R17xe2x80x2, R17xe2x80x3 and R17xe2x80x3xe2x80x2 may be identical or different between R15 and R16, between R15xe2x80x2 and R16xe2x80x2, between R15xe2x80x3 and R16xe2x80x3, and between R15xe2x80x3xe2x80x2 and R16xe2x80x3xe2x80x2, respectively, and each is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms; R18 is hydrogen or a monovalent hydrocarbon group of 1 to 15 carbon atoms containing a carboxyl or hydroxyl group; R19 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing at least one partial structure selected from the group consisting of ether, aldehyde, ketone, ester, carbonate, acid anhydride, amide and imide; R20 is a polycyclic hydrocarbon group of 7 to 15 carbon atoms or an alkyl group containing a polycyclic hydrocarbon group; R21 is an acid labile group; k is 0 or 1; x1, x2, x3, a, b, c and d represent a molar compositional ratio of the recurring units associated therewith, satisfying x1+x2+x3+a+b+c+d=1, x1, x2, x3, a, b and c are numbers inclusive of 0, d is a number of more than 0, all of x1, x2 and x3 are not equal to 0 at the same time. Preferably, the acid labile group represented by R21 in formula (1) is a tertiary alkyl group having a cyclic structure.
In a second aspect, the invention provides a resist composition comprising the polymer defined above.
In a third aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition defined above onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask; and optionally heat treating the exposed coating and developing it with a developer.
The polymer comprising recurring units of formula (1) is a polymer based on a (meth)acrylic acid derivative, which is copolymerized with at least one of a vinyl ether compound, an allyl ether compound, a norbornene derivative containing an oxygen atom within its ring, and a tetracyclododecene derivative containing an oxygen atom within its ring. As pointed out above, polymers of (meth)acrylic acid derivatives have good sensitivity and resolution, but poor etching resistance, partly because of many ester bonds and a low carbon density. By changing some of the copolymerized units to the above-mentioned units, the polymer of the invention has succeeded in improving etching resistance without sacrificing the sensitivity and resolution characteristic of the (meth)acrylic acid derivative polymers. And the inventive polymer drastically suppresses the occurrence of undesired phenomena including developer repellency, swelling upon development, and pattern disruption as often found when alicyclic rings are introduced into the backbone. Therefore, a resist composition using the inventive polymer as a base resin satisfies all the performance factors of sensitivity, resolution and etching resistance, is fully restrained from swelling upon development, and is thus very useful in forming micropatterns.
It is noted that hybrid polymers analogous to the inventive polymer are known in the art, including hybrid polymers in which alicyclic olefin compound (e.g., norbornene)-maleic anhydride alternating copolymers are copolymerized with (meth)acrylic acid derivative units and hybrid polymers in which vinyl ether-maleic anhydride alternating copolymers are copolymerized with (meth)acrylic acid derivative units. It has been proposed to use these hybrid polymers as the base resin in resist compositions. These hybrid polymers contain maleic anhydride as an essential unit. However, the maleic anhydride units can adversely affect etching resistance on account of a high oxygen content, and react with water within the system during shelf storage, with the performance of the resist composition being markedly degraded by this ring opening reaction. The inventive polymer, which is obtained by introducing a vinyl ether compound, an allyl ether compound and an oxygen-containing alicyclic olefin compound into a (meth)acrylic acid derivative polymer, rather than introducing maleic anhydride, is not only improved in sensitivity, resolution and etching resistance, but also has excellent storage stability.